The present invention relates, in general, to the control of solid electrolyte sensors used to detect the level of oxygen in a gaseous environment and, in particular, to the control of oxygen sensors, containing a pump-able sealed internal reference chamber.
A conventional solid electrolyte oxygen sensor is described in U.S. Pat. No. 6,177,001 ('001 Patent). FIGS. 5 and 6 show a conventional sensor 2 formed of a solid oxide material, typically zirconia that includes a tubular shell 4 that is closed at one end. The shell forms a cylindrical chamber 6 that is sealed, for example, by having a plug 8 at the open end of the tube shell. The outside surface 10 of the oxide tubular shell is coated with porous platinum to create an outer electrode exposed to gases external to the sensor, such as in a heated environment 14. The cylindrical inside surface 12 of the oxide shell is coated with porous platinum to create an inner electrode exposed to the gas in the chamber 6. The inner and outer platinum electrodes and the solid oxide material separating them comprise a single cell oxygen sensor 2 that functions according to the Nernst principle when the cell is operated at an elevated temperature, typically greater than 700° C. The sensor 2 is typically mounted in an oven 14 or other high temperature environment.
A common mode of operation of the sensor 2 is to provide a reference gas of known oxygen partial pressure, typically air, to one of the two electrode surfaces, e.g., the inside surface 12 of the shell 4. A process gas with unknown oxygen partial pressure is provided to the second electrode surface, e.g., the outside surface 10 of the shell 4. The relationship of the voltage output of the sensor to an imbalance in the two oxygen partial pressures is defined by the Nernst equation:
      E    12    =            RT              4        ⁢        F              ×          ln      ⁡              (                  P1          P2                )            
Where: E12 is the developed electromotive force; R is the universal gas constant; T is the absolute temperature; F is the Faraday constant; P1 is the process gas oxygen partial pressure, and P2 is the reference gas oxygen partial pressure. By proper manipulation of the Nernst equation the sensor can be made to give an indication of the oxygen partial pressure in an unknown gaseous environment.
The reference gas is contained within the chamber 6 defined by the inside surface 12 of the shell and the plug 8 that seals the gas into the shell. A lead wire 16 is passed through the plug and affixed to the inner electrode surface. A second lead wire 18 is affixed to the outer electrode surface 10. The inner and outer leads 16, 18 form electrical connections between the sensor 2 and a suitable control circuit 20.
When the sensor 2 is in a high temperature environment 14, oxygen ions can be made to flow though the temperature activated solid electrolyte in response to the application of a pumping current to the porous platinum electrodes of the inner and outer surfaces 12, 10 of the tubular shell 4. The polarity of the applied current determines the direction of the ionic oxygen flow with said flow being in opposition to the applied current polarity. In this manner the oxygen partial pressure in the sensor sealed internal reference chamber 6 can be substantially altered as a function of the current applied to the electrode surfaces 10, 12.
The pumping current may be applied in discrete amounts, or pulses, to remove oxygen from the sealed reference chamber until the chamber is determined to be effectively empty as indicated by the Nernst voltage reaching a predetermined value. In a further step, the pumping current polarity is reversed and pulses are applied to the electrodes 10, 11 to cause oxygen to flow from a prevailing external gaseous environment, typically a gas with known oxygen partial pressure such as air, into the previously emptied sealed internal reference chamber. In particular, the application of pumping current is in a pulsed mode comprising in the first instance, a pulse with controlled height and width and in the second instance, a measurement interval during which no pumping current is applied to the sensor but during which a sensor voltage reading is taken to determine the level of oxygen in the sealed internal chamber 6 in relation to the level of oxygen in the prevailing external environment. The application of this two-step, pump-measure process continues until the measured output voltage reaches a predetermined value, typically zero volts or null. At this null state, the partial pressures of oxygen at both electrode faces are substantially equivalent. By integrating the pumping current required to transition the internal reference chamber from the empty state to this null or balanced state, the relationship between the total applied charge and the quantity of oxygen transferred can be calculated and stored as a sensor calibration factor.
The pulse based pumping method may be used to cause the sensor internal reference chamber oxygen partial pressure to substantially track a varying, external, unknown gaseous environment oxygen partial pressure by applying current pulses of the appropriate polarity so as to cause the transfer of oxygen into or out of the internal reference chamber such that the two partial pressures remain substantially at null or in balance as indicated by a sensor voltage reading close to zero volts.
By integrating the pumping current required to maintain the null or balanced state an accumulated charge value might be ascertained. This charge value, in conjunction with the aforementioned calibration factor, may be used to calculate the actual oxygen partial pressure inside the internal reference chamber. It follows that this calculated internal partial pressure in conjunction with the measured sensor voltage may be used to calculate an instantaneous oxygen partial pressure value for the external unknown gaseous environment.
With respect to the sensor shown in FIGS. 5 and 6 there is a potential that, due to manufacturability and aggressive external process measurement conditions, leakage paths may negatively affect the ability of the sensor system to cause the sensor internal reference chamber to substantially track and remain quantifiably in balance with the external gaseous environment under investigation. Further, the sensor pumping system described above has the added disadvantage that in situations of very low oxygen partial pressures, whether due to low partial pressures in the external gaseous environment, low partial pressures in a calibration gas, or the reference and external partial pressures being substantially close to a low partial pressure null point, the amount of charge intended to perform a specific pumping action may be greater than the quantity of oxygen available to be pumped thereby causing a potential oscillatory state in the pumping mechanism and/or causing the excess pumping current to disadvantageously polarize the sensor cell. It is therefore desirable to provide an improved pumping method capable of performing a sensor leakage check routine. It is further desired to provide an improved pumping method capable of operation in very low oxygen partial pressure environments.